Gas compressor distribution systems, particularly air compressor systems are used in a wide variety of applications to furnish compressed air to a distribution system composed of a header, sometimes including a storage reservoir, where there are multiple takeoffs from the header so compressed air is supplied to a variety of locations in a facility. In many applications the air used to operate numerous pieces of pneumatic machinery. It is a characteristic of most such machinery that the most efficient operation is achieved which the air is supplied at a uniform pressure. This is because the compressor operates in response to mass flow rate and-not volume flow rate even though compressors are typically rated according to volume flow rate. If a large demand is created, for example by opening a relatively large outlet, without control, the compressor discharge pressure increases to accommodate the increased flow and particularly inefficient operation results because the compressor is required to supply air at higher pressure so the mass flow rate is greatly increased. On the other hand if the supply pressure is controlled before the discharge, then the compressor generates air at a lower mass flow rate but still is able to accommodate the consumption needs but the energy requirements are substantially reduced because of the lower mass flow rate.
Accordingly it is important to control the pressure of the air supply to the header as closely as possible to provide air to the takeoffs at optimum pressure and minimum mass flow rate.
Many air supply systems are equipped with a multiplicity of compressors where the compressors are cycled on and off as needed to maintain the required air flow at the required pressure.
While such systems can operate satisfactorily but no prior art system is known to anticipate the relative capacity left in the operating compressor so the supplemental compressors do not come on line until a time after the operating compressors have been fully extended so there is a period of lower than desired line pressure. As a result, many facilities operate the auxilary compressor continuously even though it is unloaded to avoid the problem. This of course significantly increases the energy consumption of the system.
While it is possible to maintain excess pressure from the compressor to the header considerable difficulty has been experienced in finding the means to effectively reduce the pressure to the desired supply pressure and maintain the desired pressure to the distribution system over a wide range of demand rates.
Another problem encountered with systems operating pneumatic equipment is that even momentary interruption of the required air pressure can lead to the production of defective or off specification product and other equally undesirable problems.
Additionally, the cost of energy required to operate compressors to supply compressed air continually increases so that it is necessary to conserve air and operate air supply systems at optimum efficiency to maintain control of cost in many applications.
Method and apparatus for dealing with problems associated with maintenance of air pressure in distribution systems has been encountered in the prior art.
In one method pressure regulating valves are used which measure the air pressure in the header and through a series of mechanical devices such as diaphragms and springs operate a valve to attempt to adjust the pressure in the header. Such methods have proven ineffective for several reasons including the simple fact that the mechanical feedback system is incapable of responding quickly and accurately to pressure change, particularly where pressure may change rapidly and frequently as normally encountered in air systems operating pneumatic equipment. Additionally, such systems have not been capable of providing the air flow rates frequently required in operation of air distribution systems.
A similar problem in encountered in systems where a control valve is provided in the header upstream of the first takeoff with an actuator to operate the valve to modulate flow to the header, a pressure transducer to measure the pressure of the gas in the header between the valve and the first takeoff and a controller to receive a signal from the pressure transducer and generate a controller signal corresponding to a valve position required to maintain the flow of gas to the header to maintain the selected pressure where a position control device is provided to receive the controller signal and generate a pneumatic control signal in response to the controller signal and a mechanically generated position feedback signal from the actuator where the pneumatic control signal is supplied to an operator to operate the actuator to move the valve in a corrective direction. Because of the mechanical linkages and the pneumatic signals required such systems and apparatus have generally proved ineffective and insufficiently responsive to changes in demand to effectively control pressure in the distribution system.
Another prior art arrangement for accomplishing the objective of controlling air distribution systems is provided by "Conservair Technologies" of Kenosha, Wis. which provides two or more air circuits operating in parallel flow where "ON-OFF" valves are provided in each circuit. The valves are cycled between on and off as needed to attempt to maintain air pressure as needed in the header. In many cases several control locations may be required and the ability of the system to maintain pressure is limited by the flow characteristics of each of the parallel control circuits.
In general, no prior art arrangement or method is known which provides the effectiveness, responsiveness and advantages of devices within the scope of the present invention.